1. Field of the Invention
The present invention relates to an image display medium that uses particles and can repeatedly display images and to an image formation device.
2. Description of the Related Art
Heretofore, display technologies such as a Twisting Ball Display (display by rotation of particles that are coated with two separate colors), electrophoresis, magnetophoresis, thermally rewritable display media, and liquid crystals having a memory property have been proposed as repeatedly rewritable display mediums. However, these image display technologies, although excellent in a memory property of images, have problems in that they are unable to display white as ordinary paper does and that contrast is low.
A display technology, in which a conductive colored toner and white particles are contained in space between opposing electrode substrates, and electric charges are injected through a charge transport layer disposed on the inside surface of the electrode of a non-display substrate to the conductive colored toner, and an electric field between the electrode substrates causes charge-injected conductive colored toner to move toward a display substrate located facing the non-display substrate, and the conductive colored toner sticks to the inside of the display substrate, and contrast between the conductive colored toner and the white particles enables display of an image, was proposed as a display technology using a toner that solves such problems as mentioned above (Japan Hardcopy '99 Papers, pp. 249–252). The display technology is excellent in that the whole image display medium is made of solid matters and that display of white and black (color) can be completely switched in principle. However, according to the technology, there are conductive colored toner particles that do not come into contact with the charge transport layer disposed on the inside surface of the electrode of the non-display substrate and conductive colored toner particles each of which exists apart from the other conductive colored toner particles. Electric charges are not injected into these lone conductive colored toner particles. Therefore, they do not move even when an electric field is applied thereto. Moreover, they are present at random between the substrates. Accordingly, density contrast lowers.
With an intention of improving such problems, an image display medium that includes a pair of substrates and plural kinds of particles that are contained in space between the pair of substrates, that can be moved between the substrates by an applied electric field and that have different colors and charging properties has been proposed (Japanese Patent Application Laid-Open (JP-A) No. 2001-312225). According to the proposal, it is described that high whiteness and density contrast can be obtained.
In the image display medium, white density, black density and density contrast are excellent at an initial stage. However, when rewriting is repeated over a long period, image density may decrease, which decrease density contrast. Moreover, uniformity of an image may decrease, which causes image irregularity. Furthermore, contamination of the transparent substrate proceeds during long-term use, which may decrease contrast, display glossiness and visibility.
After earnestly researching the aforementioned problems, the inventors of the present invention found that instability of a charge amount due to frictional charging between particles, and adhesion to the transparent display substrate of particles pulverized due to collision and grinding between particles cause these problems.
Accordingly, there is a need for an image display medium that, even after repetition of rewriting over a long period, has small image density variation and small variation in density uniformity and can display images with stable density contrast, and for an image formation device using the same.